Challenging the claims on the potential of biochar to mitigate climate change.

Thesis (PhD) - Wageningen University, Wageningen. Promotors: T. W. Kuyper, E. Hoffland, B. J. M. Arts, Wageningen University. Co-promotor: Etelvino Henrique Novotny, CNPS

Challenging the claims on the potential of biochar to mitigate climate change

Summary In this PhD thesis I studied the influence of biochar discourses on the political practices in Brazil and the impact of biochar on soil organic carbon (SOC) stocks, thus contributing to the current debate on the potential of biochar to mitigate climate change. Biochar is the solid material obtained from the carbonization of biomass. The deliberate production and application to soil distinguishes biochar from other carbonized products, e.g. charcoal. Inspired by the aged charcoal found in the fertile Amazonian Dark Earth (ADE; also known as Terra Preta de Índio), the current application of biochar in soil is claimed to simultaneously address four global challenges: food production, climate change, energy supply and waste reduction (Chapter 1). Biochar is supposed to be an absorbent and stable material, which can be used to retain nutrients in the soil, increasing agricultural productivity, while sequestering carbon over extended periods of time. Therefore, biochar is claimed to be a means to mitigate global climate change. Furthermore, if biochar is produced in a modern pyrolysis plant, it also can co-produce bio-oil and syngas that could be used as energy. And if biochar is produced by carbonization of agricultural residue, biochar may reduce the quantity of solid waste that needs to be disposed of. In Chapter 2, I analysed the policy arrangement related to biochar along the four dimensions of the policy arrangement approach, which are actors, discourse, power and rules. I focused on Brazil, which is an important player in the international biochar debate. My analysis shows that scientists in research institutions are the dominant players in the network, while policymakers, businessmen and farmers are marginally positioned. Experts from Embrapa occupy central positions and thus exercise most power in the network. Moreover, experts linked to ADE have lost prominence in the network. The cause for this reduction was the shift from the ADE/biochar to the biochar/technology discourse. The latter discourse includes different coalitions, such as: ‘climate change mitigation’, ‘improvement of soil fertility’ and ‘improving crop residue management’. Although the biochar/climate coalition is dominant at international level, it is far less prominent in Brazil. Nationally the discourses of ‘improvement of soil fertility’ and ‘improving crop residue management’ have particularly prompted actors’ relationships and practices. However, the biochar/technology discourse is not (yet) institutionalized into formal rules in Brazil. As a consequence, the country lacks an established biochar policy field. Brazilian biochar practices focus on the carbonization of the available residues into biochar and on the application of biochar in soils to increase the SOC content and consequently the fertility of these soils. In this context, in Chapter 3 I tested in the field the potential of biochar produced in traditional kilns to increase the C contents of sandy savannah soils. My results show that biochar produced in traditional kilns is less thermally altered than that produced by industrial kilns and therefore rapidly decomposes. The decomposition rate of traditionally produced biochar was higher (decomposition constant k = 0.32-1.00 year-1) than generally assumed (k = 0.0005-0.005 year-1), and higher than the decomposition of native SOC (k = 0.22 year-1). In Chapter 4 I demonstrated in a short-term laboratory experiment that oilseed-derived biochar had a similar or higher decomposition rate than native SOC. My results show that all three tested oilseed biochars decelerate the decomposition of SOC in the biochar-amended soils, with biochar richer in aromatics having a stronger negative effect than biochar richer in aliphatics. Therefore, oilseed biochar directly increases soil C stocks and indirectly raises soil C sequestration in the short term through decreasing the decomposition of native SOC. In my research, the decomposition studies were performed using 13C isotope analysis. However, the 13C isotope analysis cannot be used when the differences of 13C isotope abundance between biochar and soil are not sufficiently large. Therefore, its use can be limited. In Chapter 5, I aimed at improving the benzene polycarboxylic acid (BPCA) method. I re-designed the protocols of the BPCA method and found a better and faster way to quantify and characterize the BPCAs derived from biochar, compared to the previous protocols. The improved method was then successfully tested and implemented in a laboratory in Brazil. Combining my findings with results of the literature, I conclude (Chapter 6) that there is no evidence that biochar is a reliable way for C sequestration in sandy soils under savannah environments. Biochar decomposition is highly variable, depending on charring conditions, soil and climate: (i) biochar produced by traditional kilns is less thermally degraded than those pyrolysed by industrial kilns; (ii) in sandy soils less biochar accumulates than in clay-silt soils; and (iii) warm-dry conditions raise the decomposition of biochar. These conclusions have a direct consequence for the development of policies on biochar, because we cannot ensure that biochar will sequester the same quantity of C for the same period at different geographical regions.</p

Rapid decomposition of traditionally produced biochar in an Oxisol under savannah in Northeastern Brazil.

Soil amendment with biochar has been claimed as an option for carbon (C) sequestration in agricultural soils. Most studies on biochar/soil organic carbon (SOC) interactions were executed under laboratory conditions. Here we tested the stability of biochar produced in a traditional kiln and its effects on the stocks of native SOC under field conditions

A review and meta-analysis of mitigation measures for nitrous oxide emissions from crop residues

Crop residues are of crucial importance to maintain or even increase soil carbon stocks and fertility, and thereby to address the global challenge of climate change mitigation. However, crop residues can also potentially stimulate emissions of the greenhouse gas nitrous oxide (N$_{2}$O) from soils. A better understanding of how to mitigate N$_{2}$O emissions due to crop residue management while promoting positive effects on soil carbon is needed to reconcile the opposing effects of crop residues on the greenhouse gas balance of agroecosystems. Here, we combine a literature review and a meta-analysis to identify and assess measures for mitigating N$_{2}$O emissions due to crop residue application to agricultural fields. Our study shows that crop residue removal, shallow incorporation, incorporation of residues with C:N ratio > 30 and avoiding incorporation of residues from crops terminated at an immature physiological stage, are measures leading to significantly lower N$_{2}$O emissions. Other practices such as incorporation timing and interactions with fertilisers are less conclusive. Several of the evaluated N$_{2}$O mitigation measures implied negative side-effects on yield, soil organic carbon storage, nitrate leaching and/or ammonia volatilization. We identified additional strategies with potential to reduce crop residue N$_{2}$O emissions without strong negative side-effects, which require further research. These are: a) treatment of crop residues before field application, e.g., conversion of residues into biochar or anaerobic digestate, b) co-application with nitrification inhibitors or N-immobilizing materials such as compost with a high C:N ratio, paper waste or sawdust, and c) use of residues obtained from crop mixtures. Our study provides a scientific basis to be developed over the coming years on how to increase the sustainability of agroecosystems though adequate crop residue management

High application rates of biochar to mitigate N2O emissions from a N-fertilized tropical soil under warming conditions.

Biochar application has been suggested as a strategy to decrease nitrous oxide emissions from agricultural soils while increasing soil C stocks, especially in tropical regions. Climate change, specifically increasing temperatures, will affect soil environmental conditions and thereby directly influence soil N2O fluxes

Predicting field N2_{2}O emissions from crop residues based on their biochemical composition: A meta-analytical approach

Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N$_{2}$O). Previous meta-analyses have linked various biochemical properties of crop residues to N$_{2}$O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N$_{2}$O emissions from specific residues. Here we combine comprehensive databases for N$_{2}$O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N$_{2}$O emissions. On average, crop residue incorporation increased soil N$_{2}$O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N$_{2}$O emissions. Crop residue effects on N$_{2}$O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N$_{2}$O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N$_{2}$O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N$_{2}$O emissions, whereas mature residues with opposite characteristics had marginal effects on N$_{2}$O. The most important crop types belonging to the immature residue group – cover crops, grasslands and vegetables – are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N$_{2}$O emissions

Soil pH-increase strongly mitigated N2O emissions following ploughing of grass and clover swards in autumn: A winter field study

Emissions from crop residues contribute largely to the total estimated N2O emissions from agriculture. Since low soil pH increases N2O production by impairing the last denitrification step, liming has been suggested as a mitigation strat- egy; however, it may also increase N2O emissions by enhancing mineralization and nitrification. To gain field-based empirical knowledge, we measured N2O fluxes with an autonomous field-flux robot in limed and control plots before and after autumn ploughing of 3-year-old grass, clover grass or red clover swards under different N fertilization re- gimes. Dolomite applied before establishing the swards raised soil pHCaCl2 from ~4.8 to ~5.8 in limed plots. Higher pH halved emissions from ploughed leys despite higher soil mineral N contents. It also reduced emissions be- fore ploughing. We observed substantial N2O fluxes after ploughing, with peaks during a relatively warm wet period after freezing and higher peaks during diurnal snowmelt over frozen soil. Average N2Ofluxes were strongly positively correlated with high herbage yields in the preceding growing seasons rather than with the presence of clover. The yield-scaled average N2O fluxes were strongest in low pH soils at all yield levels; this was a true effect of soil pH on N2O, as herbage yields were not increased by liming. Here, yield-scaled flux was defined as the average N2O flux after ploughing divided by the dry matter. Fluxes in red clover plots were similar to those in grass plots, despite the lower C/N ratio and higher total amount of N in clover residues. However, clover in mixtures with grass increased yields and N2O emissions. This suggests that higher ley production enhanced microbial activity, including nitrifiers and denitrifiers, and that the pH effect on facilitating complete denitrification to N2 overrode any effect on minerali- zation and nitrification, thus resulting in N2Omitigation

Biochar: An emerging policy arrangement in Brazil?

Biochar, the solid product of pyrolysis, has emerged as a new technology and policy tool to address various environmental challenges (climate change, food production and Agricultural waste management). The concept of biochar drew its inspiration from Amazonian practices that had led to the creation of Amazonian Dark Earth (ADE): fertile soils rich in (bio)char and human artefacts. In this article, we conceptualize biochar as an emerging policy arrangement, and examine it along the four dimensions of the Policy Arrangement Approach (PAA), which are actors, discourse, power and rules. We focus on Brazil as an important player in the international biochar debate. Our analysis shows that science experts are the predominant players in the network, while policy-makers, businessmen and farmers are marginally positioned. Experts from Embrapa occupy central positions and thus exercise most power in the network. Moreover, experts linked to ADE have lost prominence in the network. The reason for this is to be found in the shift from the ADE/ biochar to the biochar/technology discourse. The latter discourse includes different coalitions such as ‘climate change mitigation’, the ‘improvement of soil fertility’ and ‘improving crop residue management’. Although the biochar/climate coalition is dominant at the international level, it is far less prominent in Brazil. Nationally, it is particularly the discourses of ‘improvement of soil fertility’ and ‘improving crop residue management’ which have prompted actors’ relationships and practices. However, the biochar/technology discourse has not (yet) been formally institutionalized in Brazil. As a consequence, the country lacks an established biochar policy field